This invention relates to a multi-piece solid golf ball in which a core, an intermediate layer and a cover are formed as successive layers. More specifically, the invention relates to a multi-piece solid golf ball which, in spite of having a soft urethane cover (outer layer) that is thinly designed, is able to maintain a good durability.
Numerous multi-piece solid golf balls of three or more pieces in which an intermediate layer is interposed between a core and a cover and the respective layers are of specified hardnesses and thicknesses have thereto been disclosed. The cover and the intermediate layer, as layers which encase the core, are important elements for lowering the spin rate of the ball on shots with a driver and providing suitable spin control on approach shots. At the same time, there is a desire to improve the adhesion between these layers.
For example, JP-A 2011-11088 describes a multi-piece solid golf ball in which a reinforcing layer that uses a carboxyl group-containing polyurethane and a polycarbodiimide (water-dispersible) is situated between the intermediate layer and the cover. The intermediate layer and cover materials in this golf ball are each formed of an ionomer resin as the base resin.
However, in this prior-art disclosure, although water-dispersible polycarbodiimides have an excellent compatibility with aqueous polyurethanes, the affinity with the ionomer resin or highly neutralized resin composition serving as the base resin decreases. Hence, adhesion between the cover and the intermediate layer can hardly be regarded as sufficient.
Recently, the use of urethane materials as the base resin in cover materials for multi-piece solid golf balls is often seen. In golf balls having a cover made of a urethane material, when an ionomer resin or a highly neutralized resin composition is used as the base resin of the intermediate layer, there is a desire for good affinity or adhesion between the intermediate layer and the urethane cover. In particular, there exist recently many golf balls in which the outermost cover layer has a small thickness. Even when a soft urethane cover (outer layer) is designed so as to be thin, there is a strong desire for the ball to be capable of maintaining a good durability on repeated impact while retaining a good spin performance on approach shots.